Aqueous resin, which has solubility or dispersibility in water, finds a variety of uses, including coating, ink, a fiber-treatment agent, an adhesive, and a coating agent.
In recent years, there has arisen a sharp demand for such water-soluble or water-dispersible aqueous coating materials employing an aqueous medium, since they do not cause environmental pollution or fire, and coaters (e.g., brush, roller, and spray gun) and remaining coating materials can be readily cleaned by use of water.
For producing aqueous resin having solubility or dispersibility in water, a carboxylic group is generally incorporated into the resin. However, carboxylic groups remaining in the formed coating film cause hydrolysis of the resin, and in some cases, the strength, durability, and appearance of the coating film are impaired.
In order to enhance physical properties (e.g., strength, durability, and appearance) of coating film formed from such an aqueous resin, external cross-linking agents such as an aqueous melamine resin, an aziridine compound, and a water-dispersed isocyanate compound, which can form a cross-linking structure through reaction with the aforementioned carboxylic group, are generally employed.
However, these external cross-linking agents may be difficult to handle due to problems in toxicity, reactivity, etc. More specifically, cross-linking reaction involving the cross-linking agent of the above type proceeds with transformation of carboxylic groups. That is, when the number of the carboxylic groups decreases, the strength, durability, appearance, etc. of the coating film is enhanced. However, unreacted portions of the cross-linking agent remaining in the coating film may cause toxicity. When unreacted carboxylic groups remain in the coating film, water resistance and durability of the coating film are impaired. Thus, incomplete reaction of the cross-linking agent and the carboxylic groups in the aqueous resin results in various problems.
Recently, carbodiimide compounds have become of interest, since they can resolve the problem of toxicity. For example, Patent Document 1 discloses aqueous dicyclohexylmethanecarbodiimide, which exhibits excellent reactivity and storage stability and which can be readily handled as a cross-linking agent for forming aqueous resin.
The disclosed aqueous dicyclohexylmethanecarbodiimide has no toxicity and a sufficiently long pot life.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2000-7462